pgsql: Add a test module to test after-startup shmem allocations

Add a test module to test after-startup shmem allocations

The old ShmemInit{Struct/Hash}() functions could be used after
postmaster statup, as long as the allocation is small enough to fit in
spare shmem reserved at startup. I believe some extensions do that,
although we hadn't really documented it and had not coverage for it.
The new test module covers that after-startup usage with the new
ShmemRequestStruct() functions.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6409994c7dd8cc8c06c00882880e04cc895c28bd

Modified Files
--------------
src/test/modules/Makefile                          |   1 +
src/test/modules/meson.build                       |   1 +
src/test/modules/test_shmem/Makefile               |  24 +++++
src/test/modules/test_shmem/meson.build            |  33 +++++++
.../modules/test_shmem/t/001_late_shmem_alloc.pl   |  49 ++++++++++
src/test/modules/test_shmem/test_shmem--1.0.sql    |   9 ++
src/test/modules/test_shmem/test_shmem.c           | 101 +++++++++++++++++++++
src/test/modules/test_shmem/test_shmem.control     |   3 +
src/tools/pgindent/typedefs.list                   |   1 +
9 files changed, 222 insertions(+)